tap: set IFF_ONE_QUEUE per default

historically the kernel queues packets two times. once
at the device and second in qdisc. this is believed to cause
interface stalls if one of these queues overruns.

setting IFF_ONE_QUEUE is the default in kernels >= 3.8. the
flag is ignored since then. see kernel commit
5d097109257c03a71845729f8db6b5770c4bbedc

Signed-off-by: Peter Lieven <[email protected]>
Acked-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>

diff --git a/net/tap-linux.c b/net/tap-linux.c
index a9531892a61a1e1db0c9351c93b882880968766d..36c09e24d8a697841563aba1ad6e72111d9d3388 100644 (file)
--- a/net/tap-linux.c
+++ b/net/tap-linux.c
@@ -42,6 +42,7 @@ int tap_open(char *ifname, int ifname_size, int *vnet_hdr,
     struct ifreq ifr;
     int fd, ret;
     int len = sizeof(struct virtio_net_hdr);
+    unsigned int features;
 
     TFR(fd = open(PATH_NET_TUN, O_RDWR));
     if (fd < 0) {
@@ -51,9 +52,12 @@ int tap_open(char *ifname, int ifname_size, int *vnet_hdr,
     memset(&ifr, 0, sizeof(ifr));
     ifr.ifr_flags = IFF_TAP | IFF_NO_PI;
 
-    if (*vnet_hdr) {
-        unsigned int features;
+    if (ioctl(fd, TUNGETFEATURES, &features) == 0 &&
+        features & IFF_ONE_QUEUE) {
+        ifr.ifr_flags |= IFF_ONE_QUEUE;
+    }
 
+    if (*vnet_hdr) {
         if (ioctl(fd, TUNGETFEATURES, &features) == 0 &&
             features & IFF_VNET_HDR) {
             *vnet_hdr = 1;
@@ -78,8 +82,6 @@ int tap_open(char *ifname, int ifname_size, int *vnet_hdr,
     }
 
     if (mq_required) {
-        unsigned int features;
-
         if ((ioctl(fd, TUNGETFEATURES, &features) != 0) ||
             !(features & IFF_MULTI_QUEUE)) {
             error_report("multiqueue required, but no kernel "

diff --git a/net/tap-linux.h b/net/tap-linux.h
index 65087e1419de9873a2135734f4c1ac452bc154a4..1cf35d41bd843f1732ea3d0c3ebe0f60b44a7079 100644 (file)
--- a/net/tap-linux.h
+++ b/net/tap-linux.h
@@ -34,10 +34,11 @@
 #endif
 
 /* TUNSETIFF ifr flags */
-#define IFF_TAP                0x0002
-#define IFF_NO_PI      0x1000
-#define IFF_VNET_HDR   0x4000
-#define IFF_MULTI_QUEUE 0x0100
+#define IFF_TAP          0x0002
+#define IFF_NO_PI        0x1000
+#define IFF_ONE_QUEUE    0x2000
+#define IFF_VNET_HDR     0x4000
+#define IFF_MULTI_QUEUE  0x0100
 #define IFF_ATTACH_QUEUE 0x0200
 #define IFF_DETACH_QUEUE 0x0400